1. Field of the Invention
The present invention relates to an image forming apparatus using a semiconductor laser, such as a laser beam printer, and a light quantity control device for use in such an image forming apparatus.
2. Description of the Related Art
The relation between an electric current supplied to a laser diode and a light output differs according to the laser diode, and also varies depending on the heat evolution of the laser diode itself. Thus, a mere constant current control of an open loop type cannot result in laser light emission with constant light quantity. It is necessary to monitor the output power of light and control it so as to obtain the desired output power level. This type of control is called automatic power control (APC).
FIG. 1 shows a laser control circuit which performs the APC. This laser control circuit is composed of a current stabilizer circuit 1, a switching circuit 2, an amplifier 3, and a control device 10. The current stabilizer circuit 1 is a voltage-current converter which flows an electric current I.sub.1 corresponding to a light quantity signal 4 from the control device (CPU) 10. A circuit for switching it with a laser lighting signal 5 is the switching circuit 2. Responsive to this action of the switching circuit 2, a laser diode 6 emits light. The quantity of this light emission is taken out by a photodiode 7 as a current volume, and converted by a resistor 8 into a voltage signal. The quantity of light emission taken out as a voltage volume is amplified by the amplifier 3 to become an emission volume signal 9. The control device 10 raises the level of the light quantity signal 4 while monitoring the emission volume signal 9.
A flow chart of FIG. 2 illustrates this action of the APC. This control is performed in the following manner: Before recording of one page is initiated, e.g. during preliminary rotation, a laser forced lighting signal is activated, and then the emission volume signal 9 is monitored. If the quantity of light emission is lower than the desired value, the level of the light quantity signal is raised by one step. If it is higher than the desired value, by contrast, the level of the light quantity signal is lowered by one step. If the quantity of light emission is identical with the desired value, the APC action is completed. During this action, a laser beam scans portions corresponding to the arrows of FIGS. 3 and 4.
The APC may be performed in several ways: It is performed first of all before image formation as shown in FIG. 3 (initial APC). In case a plurality of sheets are recorded continuously, it is also performed at a portion between a preceding sheet 11 and a succeeding sheet 11 of continuously fed sheets (sheet interval APC). Alternatively, the APC may be performed outside the image area as shown in FIG. 4. This method is carried out in order to ensure the light quantity level for each line and to avoid possible influences on image formation that are exerted when lines ascribed to light emission at the portion between the preceding sheet 11 and the succeeding sheet 11 are developed.
In the present specification, the initial APC refers to APC in which the semiconductor laser emits light continuously on a plurality of scanning lines. This type of APC is usually carried out during preliminary rotation in electrostatic recording. This is a way of light quantity control as a countermeasure against the disadvantage that a long time is required until a predetermined quantity of light is obtained if the quantity of light is increased stepwise from the initial value. The sheet interval APC refers to APC in which the semiconductor laser emits light on each scanning line in a non-image area, i.e., intermittently; unless otherwise indicated, this type of APC is performed at a portion between the preceding sheet and the succeeding sheet.
This method is performed preferably particularly for avoiding influences on image formation in the following cases: When laser light is emitted continuously on a plurality of lines at a portion between sheets as shown in FIG. 3, the lines are developed and adhered on the transfer roller, making the back of the sheet dirty. Moreover, both surfaces of sheets may be printed, or reverse printing may occur.
With this method, the quantity of light can be changed by one step to several steps according to a single laser lighting. Hence, a current volume set by the initial APC is used as the initial value, and the quantity of light is increased or decreased by one step on the basis of this initial value.
This type of APC which is performed by the lighting of laser light for each line need not be carried out at the interval between the sheets. It can be performed for image write scanning line, or before the recording of the first page.
Namely, the initial APC results in the continuous lighting of the semiconductor laser, whereas the sheet interval APC allows the semiconductor laser to repeat light emission for each scanning line.
As is well known, light emission from the laser diode 6 has a droop characteristic. Thus, continuous light emission leads to changes in the quantity of light emission over time. The initial APC involving continuous light emission causes fluctuations in the quantity of light emission over time, so that the quantity of light measured via the photodiode 7 takes a low value. This makes great the quantity of current volume which has been set by the control device 10 and applied to the laser diode 6 for image formation of the first page.
In the sheet interval APC, on the other hand, light emission is performed on each scanning line. Hence, light emission is completed before the quantity of light emission fluctuates, and then reemission of light takes place on a succeeding scanning line. This procedure is repeated for scanning lines. Thus, the quantity of light measured in the sheet interval APC is stabilized, and the influence of the droop characteristic of the laser diode 6 is avoided. Consequently, even if the applying current is set so as to reemit the diode with the same emission volume, there will be a difference between the emission volume after performing the initial APC and the emission volume after performing the sheet interval APC after a forced lighting signal is activated. More concretely, when the semiconductor laser is driven, the temperature of the junction varies owing to the inside transient heat resistance. At the first lighting, the temperature is low, and the efficiency of light emission is high, but as the temperature of the junction increases due to heat evolution, the efficiency of light emission decreases. Accordingly, in the case of continuous light emission, as in the initial APC, the efficiency of light emission is lower than in short-term light emission as in the sheet interval APC. Even if the same driving current is applied, the quantity of light emission differs. In case the driving current is set so as to give the same desired light quantity between the initial APC and the sheet interval APC, the quantity of light during image formation depends on which of the two types of APC has set the driving current.
As a result, when a plurality of sheets are to be recorded consecutively, the quantity of light emission for image formation on the second and later pages that is set based on the sheet interval APC is less than the quantity of light emission for image formation that is set based on the initial APC at the time of printing the first page. Thus, the image density of the first page tends to deepen compared with the image density of the second and later pages.